Cyclobutyl pyrimidine dimers are a major product of UV (200-300 nm) radiation of DNA in vivo and in vitro; they are also a major source of biological damage induced by ultraviolet light. The photoreactivating enzyme repairs UV-irradiated DNA by the specific monomerization of pyrimidine dimers in a two-step reaction: the enzyme binds to the dimer containing DNA, then the enzyme-DNA absorbs a photon in the range 300-600 nm, breaking the dimer and restoring the biological integrity of the DNA. We shall isolate large quantities of photoreactivating enzyme from E. coli and human cells, characterize the apoprotein and cofactor of the enzyme and study its function. We shall study deficiencies of photoreactivating enzyme in cells from patients with sun-light induced skin cancer, and shall determine the molecular cause of these deficiencies.